The present invention relates generally to computed tomography (CT) scanners which produce images from X-ray attenuation measurements, and particularly to techniques for removing artifacts from such images.
As shown in FIG. 1, a CT scanner for producing images of the human anatomy includes a patient table 10 which can be positioned within the aperture 11 of a gantry 12. A source of highly collimated X-rays 13 is mounted within the gantry 12 to one side of its aperture 11, and one or more detectors 14 is mounted to the other side of the aperture. The X-ray source 13 and detectors 14 revolve about aperture 11 during a scan of the patient to obtain X-ray attenuation measurements from many different angles.
A complete scan of the patient is comprised of a set of X-ray attenuation measurements which are made at different angular orientations of the X-ray source 13 and detector 14. The gantry may stop or continue to move as the measurements are being made. Each such set of measurements is referred to in the art as a "view" and the results of each such set of measurements is a transmission profile. As shown in FIG. 2, the X-ray source 13 produces a fan-shaped beam which passes through the patient and impinges on an array of detectors 14. Each detector 14 in this array produces a separate attenuation signal and the signals from all the detectors 14 are separately acquired to produce the transmission profile for the indicated angular orientation. The X-ray source 13 and detector array 14 continue to revolve in direction 15 to another angular orientation where the next transmission profile is acquired.
As the data are being acquired for each transmission profile, the signals are sampled, filtered and stored in a computer memory. The signals from the detectors are over sampled to provide twice the number of transmission profiles as are required to reconstruct an image, for example. The attenuation measurement samples then are digitally low-pass filtered and the output of the filtering is sampled at a rate that produces the required number of transmission profiles from which to reconstruct an image. These steps are performed in real time as the data is being acquired.
The resultant transmission profiles then are used to reconstruct an image which reveals the anatomical structures in a slice taken through the patient. The prevailing method for reconstructing image is referred to in the art as the filtered backprojection technique. The attenuation measurements are converted to integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a CRT display.
Each X-ray detector 14 comprises a scintillator and a solid state photodiode. X-rays striking the scintillator produce light photons which are absorbed by the photodiode creating an electric current. The light is not emitted by the scintillators instantaneously, rather the emission follows a multi-exponential curve. Similarly the light emission does not terminate immediately when the X-ray beam is cut off, but follows a multi-exponential curve. The time dependence of the emitted light intensity can be modelled accurately as a sum of several exponential terms with different decay constants.
Because the detector array is rotating rapidly about the patient, the exponential response blurs together detector readings for successive views producing an image artifact referred to as "afterglow". The afterglow degrades the azimuthal component of the image resolution which produces shading and arc shaped artifacts in the reconstructed image. The azimuthal direction 16 of the image area is perpendicular to a line 17 from the center of the imaging aperture 11.
FIG. 3 plots attenuation values from a given detector for a series of views and graphically depicts the blurring. The solid line represents the output of a single detector 14 during several views for a square object being imaged. Ideally the detector data should have a pulse-like shape as represented by the dashed lines. However, the effect of the afterglow rounds the edges of the waveform and extends the object signal into several adjacent views. When the views are used to reconstruct an image, the object will appear enlarged and will not have sharp, distinct edges. In addition, if the response of individual detectors vary (e.g. an adjacent detector may have a response shown by line 18), an inconsistence will be created in the projection which leads to arc artifacts.
An obvious solution to the resolution degradation and artifacts is to slow the rotational speed of the X-ray source and detectors and provide a delay between views that is long enough for the afterglow to decay to a negligible level. This prolongs image acquisition making the process more susecptible to patient motion artifacts. Heretofore, an minor amount of degradation has been tolerated, but as rotational scan periods become shorter, approaching one second for example, the image degradation reaches unsatisfactory levels.